Metal（iOS）中的多重采样/锯齿状边缘

Posted 2023-02-15

【中文标题】Metal（iOS）中的多重采样/锯齿状边缘

【英文标题】：Multisampling/jagged edges in Metal (iOS)

【发布时间】：2016-07-13 15:19:34

【问题描述】：

我目前正在尝试绘制一个将在 Swift 中使用 Metal 进行动画处理的图形。我已经成功地绘制了我的图形的单帧。图形很简单，正如您从这张图片中看到的那样。我不知道如何对绘图进行多重采样。总体而言，似乎很少有关于 Metal 的参考资料，尤其是在 Swift 语法方面。

self.metalLayer = CAMetalLayer()
self.metalLayer.device = self.device
self.metalLayer.pixelFormat = .BGRA8Unorm
self.metalLayer.framebufferOnly = true
self.metalLayer.frame = self.view.frame
self.view.layer.addSublayer(self.metalLayer)

self.renderer = SunRenderer(device: self.device, frame: self.view.frame)

let defaultLibrary = self.device.newDefaultLibrary()
let fragmentProgram = defaultLibrary!.newFunctionWithName("basic_fragment")
let vertexProgram = defaultLibrary!.newFunctionWithName("basic_vertex")

let pipelineStateDescriptor = MTLRenderPipelineDescriptor()
pipelineStateDescriptor.vertexFunction = vertexProgram
pipelineStateDescriptor.fragmentFunction = fragmentProgram
pipelineStateDescriptor.colorAttachments[0].pixelFormat = .BGRA8Unorm
pipelineStateDescriptor.colorAttachments[0].blendingEnabled = true

pipelineStateDescriptor.colorAttachments[0].rgbBlendOperation = MTLBlendOperation.Add
pipelineStateDescriptor.colorAttachments[0].alphaBlendOperation = MTLBlendOperation.Add
pipelineStateDescriptor.colorAttachments[0].sourceRGBBlendFactor = MTLBlendFactor.SourceAlpha
pipelineStateDescriptor.colorAttachments[0].sourceAlphaBlendFactor = MTLBlendFactor.SourceAlpha
pipelineStateDescriptor.colorAttachments[0].destinationRGBBlendFactor = MTLBlendFactor.OneMinusSourceAlpha
pipelineStateDescriptor.colorAttachments[0].destinationAlphaBlendFactor = MTLBlendFactor.OneMinusSourceAlpha

问题是，如何平滑这些边缘？

更新：

所以我实现了一个 MultiSample 纹理并将 sampleCount 设置为 4。我没有发现任何区别，所以我怀疑我做错了什么。

最终：

因此，最终，多重采样确实有效。最初，我用 0 alpha 包裹这些“射线”的顶点。这是使边缘更平滑的技巧。有了这些顶点，多重采样似乎并没有改善边缘。当我恢复到每条光线有 4 个顶点时，多重采样改善了它们的边缘。

let defaultLibrary = self.device.newDefaultLibrary()
let fragmentProgram = defaultLibrary!.newFunctionWithName("basic_fragment")
let vertexProgram = defaultLibrary!.newFunctionWithName("basic_vertex")
        
let pipelineStateDescriptor = MTLRenderPipelineDescriptor()
pipelineStateDescriptor.vertexFunction = vertexProgram
pipelineStateDescriptor.fragmentFunction = fragmentProgram
pipelineStateDescriptor.colorAttachments[0].pixelFormat = .BGRA8Unorm
pipelineStateDescriptor.colorAttachments[0].blendingEnabled = true
pipelineStateDescriptor.sampleCount = 4
        
pipelineStateDescriptor.colorAttachments[0].rgbBlendOperation =    MTLBlendOperation.Add
pipelineStateDescriptor.colorAttachments[0].alphaBlendOperation = MTLBlendOperation.Add
pipelineStateDescriptor.colorAttachments[0].sourceRGBBlendFactor = MTLBlendFactor.SourceAlpha
pipelineStateDescriptor.colorAttachments[0].sourceAlphaBlendFactor = MTLBlendFactor.SourceAlpha

pipelineStateDescriptor.colorAttachments[0].destinationRGBBlendFactor = MTLBlendFactor.OneMinusSourceAlpha
pipelineStateDescriptor.colorAttachments[0].destinationAlphaBlendFactor = MTLBlendFactor.OneMinusSourceAlpha

let desc = MTLTextureDescriptor()
desc.textureType = MTLTextureType.Type2DMultisample
desc.width = Int(self.view.frame.width)
desc.height = Int(self.view.frame.height)
desc.sampleCount = 4
desc.pixelFormat = .BGRA8Unorm
        
self.sampletex = self.device.newTextureWithDescriptor(desc)


// When rendering
let renderPassDescriptor = MTLRenderPassDescriptor()
renderPassDescriptor.colorAttachments[0].texture = sampletex
renderPassDescriptor.colorAttachments[0].resolveTexture = drawable.texture
renderPassDescriptor.colorAttachments[0].loadAction = .Clear
renderPassDescriptor.colorAttachments[0].clearColor = MTLClearColor(red: 23/255.0, green: 26/255.0, blue: 31/255.0, alpha: 0.0)
renderPassDescriptor.colorAttachments[0].storeAction = .MultisampleResolve
    
    
let commandBuffer = commandQueue.commandBuffer()
    
let renderEncoder = commandBuffer.renderCommandEncoderWithDescriptor(renderPassDescriptor)
renderEncoder.setRenderPipelineState(pipelineState)

【参考方案1】：

使用 MTKView（只需将 sampleCount 设置为视图和管道描述符上所需的 MSAA 样本数），这要简单得多，但以下是滚动您自己的步骤。

创建渲染管道状态时，将渲染管道状态描述符的sampleCount 设置为多重采样计数。

在启动时，以及每当层调整大小时，通过创建一个纹理描述符，其textureType 是MTLTextureType2DMultisample 并且其sampleCount 是您的多重采样计数，从而创建一个尺寸等于图层可绘制大小的多重采样纹理。如果您正在使用深度和/或模板缓冲区，请在其描述符上设置这些属性。

渲染时，将MSAA纹理设置为render pass描述符的原色附件texture，将当前drawable的纹理设置为resolveTexture。

将颜色附件的storeAction 设置为MTLStoreActionMultisampleResolve，以便在通道结束时将MSAA 纹理解析到渲染缓冲区中。

像往常一样绘制和呈现。

【讨论】：

请注意，在第2步中，您还必须将MSAA纹理的storageMode设置为MTLStorageModePrivate。理想情况下，您还应该将usage 设置为MTLTextureUsageRenderTarget。

使用 MTKView sampleCount 工作得很好。只需确保使用 device.supportsTextureSampleCount(count) 确定设备可以使用多少个样本，如果使用超过 1 个样本，请确保设置 renderPassDescriptor!.colorAttachments[0].storeAction = MTLStoreAction.storeAndMultisampleResolve。

如果您不需要 MSAA 缓冲区的全部内容进行进一步处理，则无需使用 .storeAndMultisampleResolve 存储操作；只需使用 .multisampleResolve 操作即可满足 99% 的情况，并且可以节省大量内存。